1. Field of the Invention
The invention relates to sunscreen compositions including a dibenzoylmethane derivative that are made more stable by the addition of low levels of an α-cyano-β,β-diphenylacrylate compound. More particularly, the invention relates to sunscreen compositions which utilize low levels and very low levels of octocrylene to stabilize other photoactive compounds present in a sunscreen composition and, in particular, to stabilize dibenzoylmethane derivatives.
2. Brief Description of Related Technology
It is well known that ultraviolet radiation (light) having a wavelength from about 280 nm or 290 nm to about 320 nm (UV-B) is harmful to human skin, causing burns that are detrimental to the development of a good sun tan. UV-A radiation (about 320 nm to about 400 nm), while producing tanning of the skin, also can cause damage, particularly to very lightly-colored or sensitive skin, leading to reduction of skin elasticity and to wrinkles. Therefore, a sunscreen composition for use on human skin preferably includes both a UV-A and a UV-B filter to prevent most of the sunlight within the full range of about 280 nm or 290 nm to about 400 nm from damaging human skin.
Ultraviolet radiation from the sun or artificial sources can also cause harm to coatings containing photoactive substances, such as photoactive pigments and dyes, by breaking down chemical bonds in the structure of a component such as a polymer, a pigment, or a dye. This photodegradation can lead to color fading, loss of gloss, and loss of physical and protective properties of a coating. Photodegradation can take place in several steps which include one or more components of a coating absorbing UV radiation. The absorbed radiation can excite the absorbing molecules and raise them to a higher energy level, which can be very reactive. If the molecule cannot be relaxed, bond cleavage and the formation of free radicals will occur. These free radicals can attack one or more color molecules and/or a polymer backbone and form more free radicals. UV-A and UV-B filters can also be used to accept UV radiation to protect a pigmented coating.
The UV-B filters that are most widely used in the U.S. in commercial sunscreen compositions are paramethoxycinnamic acid esters, such as 2-ethylhexyl paramethoxycinnamate, commonly referred to as octyl methoxycinnamate or PARSOL MCX, octyl salicylate, and oxybenzone.
The organic UV-A filters most commonly used in commercial sunscreen compositions are the dibenzoylmethane derivatives, particularly 4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane (also called avobenzone, sold under the brand name PARSOL 1789). Other dibenzoylmethane derivatives described as UV-A filters are disclosed in U.S. Pat. Nos. 4,489,057, 4,387,089 and 4,562,067, the disclosures of which are hereby incorporated herein by reference. It is also well known that the above described UV-A filters, particularly the dibenzoylmethane derivatives, can suffer from rapid photochemical degradation, when used alone or when combined with the above-described most commercially used UV-B filters.
The performance of a photoactive compound or a combination of photoactive compounds in a sunscreen composition has been extremely difficult to predict based on the levels of photoactive compounds in the formulation, particularly when the formulation includes one or more photoactive compounds that suffer from relatively rapid photodegradation, such as avobenzone. Because of this, each formulation has required expensive laboratory testing to determine the UV absorbance, as a function of time (quantity) of exposure of the formulation to UV radiation. Moreover, a particularly difficult problem is presented when one photoactive compound in a sunscreen composition acts to increase the rate of photodegradation of another photoactive compound in the composition. This can be accomplished in a number or ways, including a bimolecular reaction between two photoactive compounds and a lowering of the threshold energy need to raise a photoactive compound to its excited state. For example, when avobenzone is combined with octyl methoxycinnamate a bimolecular pathway leads to the rapid photodegradation of both the dibenzoylmethane derivative and the octyl methoxycinnamate.
Methods and compositions for stabilizing photoactive compounds, such as dibenzoylmethane derivatives, with diesters and/or polyesters of naphthalene dicarboxylic acid are described in U.S. Pat. Nos. 5,993,789, 6,284,916 and Gers-Barlag, et al. U.S. Pat. No. 6,491,901 ('901). Alternatively, Deflandre et al, U.S. Pat. No. 5,576,354 and Gonzenbach et al., U.S. Pat. No. 6,033,649 describe the use of high levels of an α-cyano-β,β-diphenylacrylate compound to stabilize a sunscreen composition including a dibenzoylmethane derivative. Thus, Deflandre et al. teach a minimum 1% by weight of octocrylene and Gonzenbach et al. teach a minimum of 0.5% by weight of octocrylene to stabilize a dibenzoylmethane derivative in a sunscreen composition. Gers-Barlag, et al. U.S. Pat. No. 6,491,901 ('901) discloses sunscreen compositions containing a dibenzoylmethane derivative with a stabilizing combination of octocrylene and diesters or polyesters of naphthalene dicarboxylic acid wherein the examples have a weight ratio of octocrylene to the diester or polyester of naphthalene dicarboxylic acid in the range of 0.16 to 0.725.